1. Technical Field
The present disclosure relates to telecommunication and, more specifically, to wireless communication.
2. Description of Related Art
The new 4G wireless technology standard termed Long Term Evolution-Advanced (LTE-A) utilizes the well-known modulation scheme known as orthogonal frequency division multiple access (OFDMA). It is a multicarrier technique in which the transmit spectrum is divided into K orthogonal subcarriers equally spaced in frequency. The method has been used for many years in both wireline broadband communications and wireless local area networks (WLAN). LTE-A provides a minimum of 1000 Mbps throughput in the downlink (DL) and 500 Mbps in the uplink (UL). The spectral bandwidth for LTE-A is 100 MHz, using up to five component carriers each with a component bandwidth of up to 20 MHz. LTE-A also includes support for both frequency domain duplexing and time domain duplexing.
LTE-A also employs multiple antenna methods such as spatial multiplexing and transmit diversity. Spatial multiplexing (SM) is a multiple-input-multiple-output system (MIMO) formulation enabled by configuring multiple antennas separated in space. The spatially separated antennas provide separate and distinct transmission channels allowing the transmitter-receiver pair to extract independent signals from each channel while cancelling interference from the other transmission paths. When combined, OFDMA and MIMO-SM provide orthogonality in both frequency and space. LTE-A supports up to eight antennas per modem.
Furthermore, LTE-A uses an advanced error correction coding scheme known as Turbo Coding. This is a channel coding method which utilizes a combination of convolutional coding and pseudo random interleaving. The PN interleaver is positioned between two constituent encoders, resulting in near-Shannon limit coding gain when combined with maximum a-posteriori (MAP) decoding.
Ordinarily a given LTE-A base station, referred to as an evolved Node B (eNodeB), is continuously transmitting in the down link (DL) direction and receiving signals in the uplink (UL) direction to/from several user equipment (UE) terminals simultaneously. Whenever a new UE enters the service area of the eNodeB or is otherwise activated (for example by powering up) the new UE searches for an active eNodeB, undergoes a synchronization process, and identifies the network in order to establish communication. The 3GPP standard specification contains several signals and messages to facilitate this process. Specifically, LTE-A contains three types of physical-layer signals which are used in order to allow each UE to synchronize to the eNodeB, including: 1) a primary synchronization signal (PSS), 2) a secondary synchronization signal (SSS), and 3) reference signals.
LTE-A uses OFDMA as the modulation scheme in the DL transmission direction. The UL transmission method is single-carrier OFDMA (SC_OFDMA), also known as discrete Fourier transform (DFT)-spread OFDMA. The DL OFDMA modulation technique utilizes N orthogonal subcarriers with a time-domain symbol length of N+Ncp samples, where Ncp is the length of a cyclic prefix (CP). The CP consists of Ncp samples copied from the end of the length N time-domain symbol and pre-appended in front of the original symbol. The baseband symbol is generated by computing an inverse fast Fourier transform (IFFT) where the frequency domain input consists of N complex quadrature amplitude modulation (QAM) data symbols and the output is N complex time-domain samples. A RF modulator converts the baseband signal to RF by QAM with the RF carrier signal.
After RF down-conversion, the UE receiver recovers the transmitted symbols using a FFT demodulator, reversing the modulation introduced in the eNodeB transmitter.
There are two possible values of OFDMA carrier spacing in LTE-A, namely 7.5 kHz and 15 kHz, which represents the spacing between each of the N carriers over the entire transmit spectrum. Several different FFT sizes N may be used depending on system configuration, including: 128, 256, 512, 1024, 2048, 4096 (with 7.5 kHz carrier spacing). Furthermore, there are three possible sub-symbol modulation specifications, namely QAM, 16QAM, and 64QAM. QAM transmits two bits per carrier using one of four possible symbols. 16QAM transmits four bits per carrier using one of sixteen possible symbols. 64QAM transmits six bits per symbol using one of sixty-four possible symbols. The length of the cyclic prefix Ncp is specified as shown in Table 1 below.
TABLE 1OFDM ParametersConfigurationCyclic Prefix Length Ncp,INormal cyclic prefixΔf = 15 kHz 160 for I = 0 144 for I = 1, 2, . . . 6Extended cyclic prefixΔf = 15 kHz 512 for I = 0, 1, . . . 5Δf = 7.5 kHz1024 for I = 0, 1, 2
LTE-A specifies a specific radio frame structure. One radio frame is a 10 ms interval, which consists of 10 subframes with a duration of 1 ms each. Each subframe is composed of two slots each with a duration of 0.5 ms. Each slot contains a number of symbols specified below. There are two possible frame structures defined by LTE-A, and they are shown below. Referring to FIG. 5, frame structure 1 corresponds to frequency domain duplexing (FDD), and frame structure 2 corresponds to time domain duplexing (TDD).
A resource grid is defined in order to facilitate signal transmission and coherent detection. The resource grid is defined over all N carriers in the transit spectrum and over symbol time in the other direction. The DL resource grid is defined in terms of resource elements and resource blocks specified as NRBDLNscRB subcarriers and NsymbDL OFDM symbols, where NRBDL is the number of resource blocks in the DL and NscRB is the number of subcarriers per resource block. NsymbDL is the number of symbols in a resource block, corresponding to one slot (½ sub-frame). With one resource element defined to consist of one carrier with a duration of one symbol, a resource block consists of NRBDLNscRB×NsymbDL resource elements. The DL resource grid is shown in FIG. 6. Resource block parameters for DL are shown in Table 2 below.
TABLE 2Physical Resource Block Parameters for Downlink (DL)ConfigurationNscRBNsymbDLNormal cyclic prefixΔf = 15 kHz127Extended cyclic prefixΔf = 15 kHz6Δf = 7.5 kHz243
Considering that LTE-A is a multi-antenna MIMO processing system, the resource grid definition as discussed above exists on the transmit signal for each antenna. LTE-A is a variable bandwidth system in which the width of the transmit spectrum varies with the number of carriers and FFT size. As the FFT size is increased, the bandwidth grows out from the DC component in a symmetrical fashion so that the DC carrier is always at the center of the system bandwidth. Both the PSS and SSS occupy carriers in a block of 62 carriers centered in the middle of the frequency band. The PSS is placed in the last symbol of slots 0 and 10 (slots numbered 0-19) and therefore separated by ½ radio frame. Each cell is associated with a cell identification (ID). There are 504 unique physical-layer cell identities given by the following expressions:NIDcell=3NID(1)+NID(2)                 where:        NID(1) is in the range of 0 to 167; and        NID(2) is in the range of 0 to 2.        
There are three different PSS sequences depending on NID(2). PSS detection provides the following: 1) symbol boundary alignment, 2) half-frame synchronization, 3) partial cell identification, 4) adjacent cell monitoring, and 5) 62-carrier frequency equalizer (FEQ) reference for SSS detection.
The sequence d(n) used for the primary synchronization signal is generated from a frequency-domain Zadoff-Chu sequence according to the following expression:
            d      u        ⁡          (      n      )        =      {                                        ⅇ                                          -                j                            ⁢                                                          ⁢                                                π                  ⁢                                                                          ⁢                                      un                    ⁡                                          (                                              n                        +                        1                                            )                                                                      63                                                                                        n              =              0                        ,            1            ,            …            ⁢                                                  ,            30                                                            ⅇ                                          -                j                            ⁢                                                          ⁢                                                π                  ⁢                                                                          ⁢                                      u                    ⁡                                          (                                              n                        +                        1                                            )                                                        ⁢                                      (                                          n                      +                      2                                        )                                                  63                                                                                        n              =              31                        ,            32            ,            …            ⁢                                                  ,            61                                              where the Zadoff-Chu root sequence index u is given according to the following:        
NID(2)Root Index u025129234
The sequence d(n) is mapped to the resource elements according to the following:
                    a                  k          ,          l                    =              d        ⁡                  (          n          )                      ,                  ⁢          n      =      0        ,    …    ⁢                  ,    61        k    =          n      -      31      +                                    N            RB            DL                    ⁢                      N            sc            RB                          2            
For frame structure type 1, the primary synchronization signal is mapped to the last OFDM symbol in slots 0 and 10.
In summary, the primary synchronization signal occupies 62 carriers centered in the middle of the frequency band, is placed in the last symbol of slots 0 and 10 (slots numbered 0-19), and is separated by ½ radio frame.